Helical waveguide

ABSTRACT

A waveguide having an internal wall formed by a plurality of insulated straps of conductive material wound on edge in a helix so that the major surfaces thereof face each other, each strap being of a different cross-section or being differently disposed so as to leave a space between adjacent major surfaces. The helix is covered by a sheath of flexible insulating material which may be covered by an armor which, in turn, may be covered by a protective layer of flexible insulating material.

United States Patent Ferrentino 51 Oct. 24, 1972 [54] HELICAL WAVEGUIDE[72] Inventor: Antonio Ferrentino,Monza, Italy [73] Assignee: IndustrlePlrelll Societa per Azioni,

Milan, Italy 22 Filed: Aug.31,197l

21 Appl.No.: 176,499

[30] Foreign Application Priority Data Feb. 17, 1971 Italy ..20647 A/7l[52] US. Cl. ..333/95 A, 333/95 [51] Int. Cl. ..H0lp 3/14 [58] Field ofSearch ..333/95, 95 A; 29/600, 601

[ 56] References Cited FOREIGN PATENTS OR APPLICATIONS 1,151,951 2/1958France ..333/95 A Primary Examiner-Herman Karl Saalbach AssistantExaminer-Marvin Nussbaum Attorney-Lorimer P. Brooks et al.

[57] ABSTRACT A waveguide having an internal wall formed by a pluralityof insulated straps of conductive material wound on edge in a helix sothat the major surfaces thereof face each other, each strap being of adifferent crosssection or being differently disposed so as to leave aspace between adjacent major surfaces. The helix is covered by a sheathof flexible insulating material which may be covered by an armor which,in turn, may be covered by a protective layer of flexible insulatingmaterial.

8 Claims, 5 Drawing Figures HELICAL WAVEGUIDE The present inventionrelates to improvements in helical waveguides with tight and closelyadjacent coils, particularly suitable, by virtue of their flexibility,to join optically misaligned points.

The invention relates particularly to helical waveguides of the typedescribed in co-pending application Ser. No. 139,436, filed May 3, 1971,in the names of Paolo Gazzana Priaroggia and Antonio Portinari andhaving a common assignee. The waveguides described therein are formedfrom insulated, helically wound conductive straps, wound on edge, andthe adjacent turns are of the same cross-sectional shape. The so-formedhelix is covered by at least a sheath of insulating material which maybe covered by an armoring layer and afurther sheath of insulatingmaterial. The major surfaces of the straps are flat or curved but arecomplementary so that continuous turns are in contact throughout suchsurfaces but are out of contact at their edge surfaces. v

The deformation of a waveguide of this kind takes place along a neutralaxis, which does not pass through the center of gravity of thewaveguide, but which passes very near the outer edge of the coils of thehelix. A waveguide of this type, although it is already very flexiblwith respect to the conventional ones, still maintai s a certainundesirable rigidity in view of the purpo s it is wished to obtain. Infact, the minimum admis. 'ble bending radius is still too great topermit the collection of the guide on easily transportable drums, eitherin the factory or in the field.

The invention according to the present application, aims at providing ahelical waveguide, improved with respect to the waveguides of saidco-pending application, which may be collected on drums of practicaldiameter, namely, on drums which can be normally transported in thefactory or on the road.

A further aim of the present addition is that of obtaining a waveguidewhich, owing to its high flexibility, does not suffer a substantialdeformation from circular to oval form when it is bent, so that itssection is not subjected to deformations other than those permitted byusual tolerances and its possible deformation to oval form is merely ofan elastic nature.

A still further aim of the present invention is that of providing animproved helical waveguide which may be subjected to considerabledeformations before giving rise to spaces between the coils in the zonesin which, when the guide is bent, the insulating sheath is stretched.

The main object of the present invention is an improved waveguide of thehelical type similar to the waveguides described in said co-pendingapplication, in that a sheath of thermoplastic material covers an innerhelix whose coils are aligned and closely adjacent to one another andconstituted by insulated conductors, in the form of straps and woundupright, but difiering therefrom in that the helix is formed from aplurality of interleaved conductive straps, alternate turns being afirst helical line and, on the other face, along at least a helical linehaving a diameter different from that of the first helical line, so asto create between the major surfaces of the turns a space permitting thefree elastic deformation of the strap at each turn thereof.

Such object and other objects of the invention will be apparent from thefollowing detailed description of presently preferred embodiments of theinvention, which description should be considered in connection with theattached drawing in which:

FIG. 1 is a fragmentary, longitudinal section of an embodiment of theguide according to the invention, in which the straps are rectilinear incross-section;

' FIG. 2 is a similar view of an embodiment of the guide according tothe invention, in which the straps are oppositely curvilinear incross-section;

. FIG. 3 is a similar view of a further embodiment of the guideaccording to the invention, in which the straps are curvilinear incross-section but of different shape;

FIG. 4 is a similar view of an embodiment of the guide according to theinvention, in which one strap is rectilinear in cross-section and theother has two rectilinear portions extending at an angle to each other;and

FIG. 5 is a similar view of a further embodiment of the guide accordingto the invention, in which one strap is rectilinear in cross-section andthe other strap is curvilinear in cross-section.

In the drawings, like reference numerals indicate like parts.

The helical waveguide 10 represented in FIG. 1 comprises a cylindricalhelix 11, preferably consisting of a strap of enamelled copper oraluminum, enclosed in a sheath 12 of thermoplastic material, as forinstance polyethylene, which is covered by a protective armor 13 and bya second thermoplastic sheath 14. The armor l3 and the sheath 14 couldhowever be omitted.

The cylindrical helix 11 comprises two straps 15 and 15'. Although ahelix having a number of straps equal to, or multiple of, two ispreferred, this does not exclude that waveguides with helices of 2n+ 1straps may be made according to the invention. The strap 15 is inclinedwith respect to strap 15. Each turn of strap 15 touches, at its lowerend, along a first helical line, the preceding turn of strap 15 and, atits upper end, along a second helical line, having therefore a diameterdifferent from the first helical line, the subsequent turn of strap 15.

In FIG. 2, the strap 15 is shaped in such a way as to have an arc of acircle curvature opposite with respect to that of the strap 15, and thebending radius of strap 15 is equal to the bending radius of strap 15'.In the embodiment of FIG. 2, the strap 15' is in contact, on one face,with the adjacent strap 15 along two helical lines, and on the otherface, with the same strap 15 along a single helical line. In theillustrated embodiment, the centers of curvature of the straps are allaligned with one another, but obviously waveguides according to theinvention could be obtained with a different alignment.

FIG. 3 illustrates helix obtained with straps l5 and 15 having differentcurvatures and arranged with the same orientation, namely with thecenters of curvature directed in the same sense. Also here, the strap15' meets, on one face, the strap 15 along a single helical line, and onthe other face again the same strap 15, but along two helical lines ofdifferent diameter. The same is true for the embodiments shown in FIGS.4 and 5. In these embodiments, the strap 15, constituting one of the twostarts, is rectilinear, i.e. has a pair of plane surfaces.

In the example of FIG. 4, the strap 15 has its crosssection constitutedby two rectilinear lengths forming an obtuse angle with each other andconnected by an arc. In FIG. 5, the strap 15 has its cross-section inthe shape of an arc of a circle, and the strap 15' is rectilinear.

The waveguides may also be formed by straps having shapes other thanthose illustrated. For example, the helix could comprise alternatingstraps with equal or different polycentric curvatures.

All the embodiments have in common the fact that they comprise multiplehelices and successive turns in contact with one another along one ormore helical lines, so that spaces s, s remain between the turns, whichpermit the free elastic deformation of the strap section in each turn.By means of this feature, when the illustrated waveguide is bent, in thecompressed zone the straps move closer together in an elastic manner,while in the stretched zone the pitch is increased and the turns enlargeelastically, without detaching.

To facilitate this latter feature and to maintain it even in the eventof unusual stretching of the sheath, it is preferable to sheath thecylindrical helix by extrusion. In this way, when the thermoplasticmaterial (for instance, polyethylene) extruded in the extruder cools andshrinks, it preloads said turns, by virtue of its longitudinalshrinkage, in a considerable measure, but does not flatten themcompletely, so that in the finished article the straps are mounted in anaxially preloaded or compressed condition. In other words, the straps,in condition of mechanical rest of the waveguide, namely, when the axisthereof is rectilinear, press elastically against each other. It istherefore possible to provide a waveguide whose elasticity is adjustedby the sheath and by the other protecting elements.

If desired, it is of course possible to preload the straps to therequired extent with other conventional means, but such means are notpreferred.

The strap of the helix can be made with the normally used materials,e.g. copper and aluminum, which will provide the elasticity necessary toobtain the flexibility requirements of the waveguide so produced. Inparticular, the straps having the shapes indicated in FIGS. 2 and 3(circular section curvature) have a variable flexibility, since thepairs of straps become stiffer and stiffer as they flatten under load.This results in easier manufacture of the waveguide since narrowpreloading tolerances are not to be taken into account during sheathing.Moreover, by virtue of the variable flexibility, the pitch differencesalong the guide axis, allowed tional forms can be ado ted for the strapwhich will assure a longitudinal elas icity for the package of turns,the straps having a circular section curvature, shown in FIG. 2, is thepreferred one, since it assures, better than the other, case ofmanufacture and the features of variable flexibility and of a constantpitch of the turns.

It is understood thatthe details of construction of the waveguideaccording to the invention may be varied as necessary, without departingfrom the principles of the invention.

What is claimed is:

1. A cylindrical waveguide comprising an inner wall formed by aplurality of interleaved insulated straps of conductive material woundin the shape of a helix, said straps being wound upright and disposedwith their major surfaces extending transversely to the axis of saidhelix, each strap having a portion at one surface thereof in contactwith an adjacent strap along at least one helical line and having aportion at the opposite surface thereof in contact with an adjacentstrap along at least a second helical line having a diameter differentfrom that of said one helical line and having portions of its majorsurfaces spaced from the major surfaces of the adjacent straps, therebyproviding spaces between the major surfaces thereof, and a sheath offlexible insulating material surrounding said helix.

2. A waveguide as set forth in claim 1, wherein each strap has twoportions at the opposite surface thereof in contact with an adjacentstrap along two helical lines each having a diameter different from thatof said one helical line.

3. A waveguide as set forth in claim 2, wherein at least one of saidstraps has, in cross-section, major surfaces which have the shape ofarcs of concentric circles.

4. A waveguide as set forth in claim 3, wherein the strap adjacent saidone strap is similarly shaped but oppositely disposed.

5. A waveguide as set forth in claim 2, wherein at least one of saidstraps has, in cross-section, two rectilinear portions extending at anobtuse angle with respect to each other and joined by an arcuateportion.

6. A waveguide as set forth in claim 1, wherein said straps are undercompression axially of said waveguide when the axis of said waveguide isrectilinear.

7. A waveguide as set forth in claim 1, wherein alternate straps havedifferent cross-sectional shapes.

8. A waveguide as set forth in claim 1, wherein alternate straps havethe same cross-sectional shape but are differently disposed with respectto each other.

1. A cylindrical waveguide comprising an inner wall formed by aplurality of interleaved insulated straps of conductive material woundin the shape of a helix, said straps being wound upright and disposedwith their major surfaces extending transversely to the axis of saidhelix, each strap having a portion at one surface thereof in contactwith an adjacent strap along at least one helical line and having aportion at the opposite surface thereof in contact wiTh an adjacentstrap along at least a second helical line having a diameter differentfrom that of said one helical line and having portions of its majorsurfaces spaced from the major surfaces of the adjacent straps, therebyproviding spaces between the major surfaces thereof, and a sheath offlexible insulating material surrounding said helix.
 2. A waveguide asset forth in claim 1, wherein each strap has two portions at theopposite surface thereof in contact with an adjacent strap along twohelical lines each having a diameter different from that of said onehelical line.
 3. A waveguide as set forth in claim 2, wherein at leastone of said straps has, in cross-section, major surfaces which have theshape of arcs of concentric circles.
 4. A waveguide as set forth inclaim 3, wherein the strap adjacent said one strap is similarly shapedbut oppositely disposed.
 5. A waveguide as set forth in claim 2, whereinat least one of said straps has, in cross-section, two rectilinearportions extending at an obtuse angle with respect to each other andjoined by an arcuate portion.
 6. A waveguide as set forth in claim 1,wherein said straps are under compression axially of said waveguide whenthe axis of said waveguide is rectilinear.
 7. A waveguide as set forthin claim 1, wherein alternate straps have different cross-sectionalshapes.
 8. A waveguide as set forth in claim 1, wherein alternate strapshave the same cross-sectional shape but are differently disposed withrespect to each other.